Quantum MIIM Diodes Beat Silicon

PORTLAND, Ore. — Operating at terahertz frequencies much higher than silicon devices while consuming less power and producing less heat, metal-insulator-metal (MIM) diodes are one of those promising technologies always just out of reach.

MIM diodes use quantum tunneling, which permits electrons to jump from one metal electrode to the other without interacting with the intervening insulator layer -- hence the power and heat reductions. So far, their development has been slow going.

Now Oregon State University (OSU) researchers claim to have invigorated the technology by adding a second insulator layer to produce an MIIM device that aims to solve the problems with MIM devices and come closer to taking the technology mainstream.

"What people have typically done is use two different metals on the top and bottom electrodes, but you are limited by the work function difference you can get between the different metals," said OSU professor John Conley in an interview with EE Times. "But by using two different insulators -- one with a large bandgap and one with a smaller bandgap -- we can get extra asymmetry that can even overshadow the asymmetry of the different metals."

The two insulator layers -- which for Conley's work was hafnium oxide and aluminum oxide -- enables what he called "step tunneling." Step tunneling allows more precise control of the diode asymmetry and thus its rectification capabilities at low voltages.

As a result, Conley sees his MIIM devices are poised to improve all sorts of electronic devices in wide use today, from liquid crystal displays to cell phones and televisions, as well as new types of devices such as infrared solar cells that convert radiant heat into electricity.

"The holy grails is an infrared antenna, which would harvest infrared energy in a special infrared solar cell," said Conley.

Next, the researchers hope to optimize their process, then tackle applications that use even more metal-insulator layers, such as transistors.

"This is a building block device -- the MIIM structure by itself is just a diode -- but we plan to put these devices in other structures to improve their performance, such as the MIMIM hot-electron transistor invented by Carver Mead back in the 1960s," said Conley.

Doctoral candidate Nasir Alimardani also contributed to the work, which was funded by the National Science Foundation (NSF), the US Army Research Laboratory, and the Oregon Nanoscience and Microtechnologies Institute.

Ok, so i'll be the first one to bite. What does this device actually do?

To me, it looks like a two terminal tunneling device with electrodes at two different work functions. The difference in work function means that the I/V behavior will be asymmetric, so it could show a weak diode like behavior, but probably still a lot different to what we are used to from semiconductor diodes.

Since there is no semiconductor involved, switching delays caused by minority carrier recombination (diffusion capacitance) should be absent. That is probabably the reason for the claim for speed. However there could be a disadvantage due to the extremely high capacitance of the MIM structure.

There are some other drawbacks too: The way quantum tunneling behaves in such a two layer devices means that there are some second order slope changes in the I/V behavior. The material used (Al2O3 and HfO2) are also prone to electron trapping. This means that there are most likely additional conduction mechanisms, which will give rise to device leakage at low bias, dielectric relaxation (1/t decaying current after switch) and noise.

So here are my questions:

What is the exact application of this device?

What are the relevant figures of merit that are optimized in respect to semiconductor diodes? How does it compare in the other ones?

How is this supposed to "beat CMOS" or lead to transistors? My understanding so far is, that this is a two- not three terminal device. So it can not replace a transistor.

This is a major technological achievement. Hope they work more to make it apply to practical devices. It will interesting to know quantitative comparision - gain in speed, saving in power, voltage level required to operate it.